In array antenna systems, the signals from various antenna elements need to be combined together to form a main beam. The beamformer includes signal processing hardware capable of changing the main beam to enhance the radar or ultrasonic images. Through the beamformer, it is possible to apply different weights and delays to the signals from each of the different antenna elements. As a result, the main beam can be made to point in a specific direction, and the undesirable side lobes can be eliminated. The signals can be weighted by multiplication of a signal with a corresponding weight. The signal delays are introduced by true-time-delay (TTD) lines, which can provide a varying amount of signal propagation lengths. Then the weighted and delayed signals from the beamformer are added together to form the beam for the array antennas.
There exist a variety of beamforming techniques available for radar or ultrasonic imaging applications. Some of the well-known beamforming techniques are conventional equal-weight beamformer, null-steering beamformer, optimal beamformer, space processing beamformer, broadband beamformer, and so on. For the broadband beamformer, the TTD lines are delaying the signals from array antenna elements, while the real value signal weights are introduced to the individual signal. Thus it can provide a true space-time processing capability. In a dynamic beamforming environment, the delays and weights are updated continuously and it results in a tremendous amount of beamforming computations.
A simple beamforming example using delays and weights is illustrated in FIG. 1. In FIG. 1, an imaging system based on pulse-echo principle is composed of scatter source, transducer element, delay and weight unit, and coherent summer. For a coherent summation of echo signals of different transducers, the received signals have to be delayed individually according to the distance from the scatter source and the amount of signal focusing being involved. The intensity of received echo signals can vary according to the incident angle between scatter and transducer elements. In order to account this, an aperture apodization is required by applying corresponding weights to different received signals. In dynamic receive beamforming, the depth of the receive focus point has to be updated continuously with requires a continuous update of the individual amount of delay and weight applied to the signals of each element.
Photonic devices, electronic processing system, or any combination thereof can perform the dynamic beamforming computations for ultra wideband array antennas. However, it is known to be very difficult to fabricate low-cost and low-loss electronic delay lines for multi-octave true-time-delay (TTD) with bandwidth greater than 5 GHz. In order to overcome this limitation, there have been extensive research efforts on low-loss, high-bandwidth, and low-cost TTD lines, which can achieve large delay range and high delay resolution simultaneously. The TTD lines are also required to be compact, reliable, scalable to a large array, bi-directional, and rapidly programmable.
For broadband beamforming applications, analog optical signal processing hardware is incrementally favored over the electronic counter part in order to provide improved radar performance and reduced system cost. Some of the known advantages of analog optical signal processing for array antenna beamformers are: 1) broadband beamforming capability, 2) low-loss delay lines by using optical fibers, 3) no electromagnetic interference, 4) separation of beamforming and antenna over large distance, and 5) single-fiber WDM link to replace multiple bundles of transmission lines. However the disadvantageous aspects of photonic processing include: 1) bulky and expensive optical components, 2) tight alignment and packaging requirements for optical hardware, and 3) inefficient RF-to-optical (and vice versa) interfaces.
Optical beams from one or more optical fibers, predominantly single-mode optical fibers, are collimated by using fiber collimators and then coupled into one or more optical fibers using other fiber collimators. A variety of other optical elements, such as optical switch, optical manifolds, polarization beam splitter, and so on, can be placed between these fiber collimator pairs.
A variable optical delay (VOD) is a programmable device that can vary the amount of optical signal delay by changing optical path lengths. Optical path length is the product of optical signal travel length and refractive index of the medium through which the lights travel. In a memory-less system, the optical signals reaches the destination target with a time delay determined by the optical path length. If we vary the optical path length, the time delay for optical signal transmission can be altered accordingly. Shorter optical path length decreases the time delay, while longer optical path length increases it. The optical path length can be varied by altering the signal travel length: L; or by changing the refractive index of the medium: n. Therefore, if OPL=n*L, where n is refractive index and L is signal travel length, then ΔOPL=Δn* L or ΔOPL=n*ΔL. Since conventional approaches are more readily implemented without changing the refractive index of the medium, conventional VODs are implemented by either shortening or elongating the distance between signal source and target destination.
VOD can be formed in many various configurations using many different technologies. VOD can provide programmable delays in either continuous or discrete steps. For the continuous VOD, the true-time-delay is controlled by the relative movement between signal source and receive target. Either signal source or receive target is attached to the mechanical moving stage in order to provide the necessary displacement.
FIGS. 3A and 3B depict two different conventional configurations of continuously variable delays. FIG. 3A shows a continuously variable delay using a straight optical path or a folded optical path. The relative movement between source and target triggers either true-time-delay or true-time-advance of optical signals. For the case of continuously variable optical delays with folded optical paths as shown in FIG. 3B, the number of folds being employed multiplies the amount of delay or advance available. On the other hand, the optical delay resolution for the number of folds degrades the folded optical paths. The mechanical actuators providing movements between source and target ultimately determines the VOD's performances such as speed, reliability, and delay range and resolution. The continuously variable optical delay is simple in principle and design, however commercial applications are typically bulky, heavy, slow, unreliable, and very sensitive to alignment.
FIG. 4 shows a discretely variable optical delay constructed by serially interconnecting optical switches and optical manifolds. The length of optical manifolds are incremented or decremented by 2n, where n is the stage number of serial interconnected pairs of optical switch and optical manifold. The total length of the optical manifold is incremented or decremented by 2n, where n is the stage number of serial interconnected pairs of optical switch and optical manifold. The total length of optical manifold determines the delay range of the discretely variable optical delay, while the shortest optical manifolds available determine the delay resolution. Two major optical components for discretely variable optical delays are add/drop optical switch (2×2 optical switch) and optical manifold. FIG. 4 illustrates a 4-bit discretely variable optical delay with four serially interconnected stages of 2×2 optical switch and optical manifold.
Optical fibers with controlled lengths and discrete add/drop optical switches implement one form of conventional VODs. It has the advantage of providing extremely long delays by using low-loss optical fiber. But such VODs require many discrete optical components to yield large footprint size, poor reliability, and high cost. The use of discrete optical components also restricts the minimum achievable optical path lengths and the VOD delay resolution. The add/drop optical switch is typically a commercially available 2×2 optical switches such as macro-mechanical optical switch, thermo-optical switches, magneto-optic switches, micro-electro-mechanical systems (MEMS), optical switches.
Other forms of conventional VOD utilizes fully integrated add/drop optical switches on the integrated optical waveguide circuits. The integrated optical waveguide circuit can monolithically combine optical switches and optical manifolds on a single substrate. However, the finite substrate size limits the number of optical switches and the length of optical manifolds. The VODs based on integrated optical waveguide circuit cannot deliver large delay range not fine delay resolution.
Conventional continuously variable optical delays are simple in principle and design, while providing a true-time-delay. However, conventional continuously variable optical delays present disadvantages such as slow speed, poor reliability, and tight alignment tolerance.
Conventional variable optical delay (VOD) for ultra wideband array antenna applications require a very large delay range and fine delay resolution simultaneously. But the conventional VOD utilizing optical fibers, add/drop optical switches, or integrated optical circuits presents serious constraints in achieving a large delay range and fine delay resolution. Due to discrete optical components and limited substrate area available for integrated optical circuits, the conventional VOD become expensive, bulky, and unreliable. The conventional VOD is not scalable to a large array and it is difficult to integrate with other analog optical signal processing hardware.
High-performance and low-cost variable optical delay (VOD) with a wide delay range and high delay resolution is essential for ultra wideband array antenna transmit/receive applications. As the analog photonic hardware, such as VOD, can significantly reduce the number of beamforming computations for the array antennas, new types of VODs are actively sought after in order to deliver compactness, high device density, reliability, low loss, bi-directional operation, high speed, wide delay range, and high delay resolution.
Therefore, VODs having smaller size, higher device density, greater reliability, less loss, high speed, high delay resolution, wide delay range, or combinations of these features as compared to conventional VODs are highly desirable.